Recently, there have been an interest in producing inexpensive, compact, light weight and reliable zoom cameras with a zoom ratios of greater than 1.5.times. that provide an excellent image quality over the entire zoom range for the objects that are located both short and long distances away.
U.S. Pat. Nos. 5,825,556 and 5,726,810 disclose zoom lenses for such a camera. These zoom lenses are very compact and have only five lens elements arranged into three moving groups: a front, positive power lens group, a middle, positive power lens group, and a rear, negative power lens group. The front and the rear lens groups move in unison-i.e., at the same speed and direction, while the middle lens group moves at a different rate. U.S. Pat. No. 5,726,810 discloses that the lens mounts for the front and rear lens groups are attached to a lens barrel and that the mount for the middle lens group slides along the guide rods mounted inside this lens barrel. U.S. Pat. No. 5,825,556 does not disclose a mechanism for moving the lens groups.
U.S. Pat. No. 4,198,129 discloses a zoom assembly with a zoom lens that has at least four lens groups. Three of the lens groups are movable. In one embodiment, two of the three movable lens groups are situated in the lens barrels that are linked and move parallel to one another. The remaining lens barrel, supporting the third lens group, is sandwiched between the two linked lens barrels and is either stationary or moves at the same rate and direction as the above described linked lens barrels. Because of this construction, the zoom assembly utilizes at least one additional stationary lens group. However, having four or more lens groups results in a large size zoom assembly and increases its cost.
In addition, in order to move lens groups relative to one another, the zoom assembly disclosed in U.S. Pat. No. 4,198,129 utilizes three stepper motors and a plurality of guide rods. If these guide rods are not aligned perfectly with respect to one another and the rest of the zoom assembly, the lens barrels will not move properly and, may even stop moving altogether. Finally, stepper motors are relatively expensive, and having three such motors not only significantly increases the cost of the zoom assembly, but also makes this zoom assembly bulky and heavy.
U.S. Pat. No. 5,194,991 discloses a zoom assembly having a zoom lens that has three moving lens groups. This zoom lens is capable of micro photography. In order to accomplish zooming from the wide angle position to the telephoto position each of the lens groups moves at a different rate with respect to the other lens groups. The zooming action is accomplished by a cam sleeve that has three differently configured cam grooves. Each of the cam grooves has segments of different slopes but the sign of the slopes does not change. Because these cam grooves require three sets of cam followers (one for each lens group), this zoom assembly is relatively difficult to align and assemble, resulting in higher cost of production.
This patent also discloses that the second lens group moves along a rod. If this rod is not aligned perfectly with respect to the rest of the zoom assembly, the lens barrels will not move properly and, may jam and stop moving altogether. Furthermore, the disclosed mechanism utilizes an additional focusing motor for focus adjustment, making this zoom assembly expensive to manufacture.
U.S. Pat. No. 5,646,790 also discloses a zoom assembly mechanism with three moving lens barrels. The patent discloses three differently shaped cam profiles that enable the three lens groups to move relative to one another. This configuration requires three sets of cam followers (each set corresponding to a different lens group), resulting in either a (i) tolerance buildup and thus a relatively expensive alignment and assembly, or (ii) in tighter tolerance requirements and, thus, higher manufacturing costs.